PV-Thermal has many uses beyond energy generation
In my last column I talked about photovoltaic (PV) modules getting hot during operation due to the fact that they absorb a great deal of solar energy that is converted to heat. In fact, more solar energy is converted to heat than electricity in a typical PV module. We’ve recorded temperatures of more than 150 degrees on the underside of the module used in our Energy Systems course at Chico State.
When a PV array is mounted on a rooftop, this heat is wasted and often ends up heating the roof of the house. This can cause more heat gain into the attic space and puts more load on your air-conditioning system in the summer. In the winter, this heat gain could be helpful, but the effect is greatest in the summer.
Therefore, a PV module is a thermal collector as well as an electricity producer. Why not harness this heat for beneficial purposes? This idea is not new, especially in Europe, where systems have already been designed and marketed. They are known as PV-Thermal, or PVT systems, and are used for water heating and space heating in homes in addition to providing on-site electricity. These systems heat a liquid or air that passes along the underside of a PV module. Similar to a conventional solar-thermal collector, the heated fluid can be used to supplement domestic hot water during all seasons or be used for space heating in the winter.
An added benefit of PVT is improved performance of the PV module as an electricity producer. Since silicon PV cells are more efficient when they are cooled, more electrical power is generated in a PVT system. This can raise the overall solar-to-electric efficiency of the system by up to 2 percent (e.g. from about 14 percent to 16 percent).
The only company in the United States I know of that offers such a system is PVT Solar in Berkeley (www.pvtsolar.com). The company’s “echo” design is somewhat different from European systems—air is drawn under the PV array, which is followed by a row of conventional solar-thermal collectors to boost the air temperature. This high-temperature air is ducted through the roof to an energy-transfer module located in the attic. This module contains the fan that pulls the air into the house, a heat exchanger for heating domestic water, and a filter for cleaning the air before it enters the house for space heating. If space heating is not needed, the air is simply exhausted to the outside.
The echo system can be used to help cool your home, too. During the nighttime, the PV modules and solar collectors cool off quite rapidly due to radiation to the night sky. Air can be circulated through the same channels and be cooled before it enters the home.
A PVT Solar echo system will yield an additional 40 percent to 50 percent of energy capture in heat and costs about 25 percent more than a conventional PV system. With it, you get electricity generation, water heating, space heating, cooling and ventilation in one package!